PROJECT SUMMARY/ABSTRACT Tremor is an uncontrolled oscillation of muscle activity that is present in many neurological disorders and can severely disrupt the execution of daily tasks. Despite its prevalence and impact on quality of life, the specific patterns and pathways of neural activity that lead to tremor remain unknown. This is especially problematic in disorders in which tremors are not only the main symptom, but also the disease itself. A case in point is essential tremor (ET), which is the most prevalent of all tremor disorders. There has been a long-standing debate about whether the cerebellum plays a key role in the generation of tremor in ET, and there are three disparate theories of how this may occur. Purkinje cell loss, GABA receptor reduction, and inferior olive oscillations have all emerged as potential mechanisms to generate tremor. This proposal seeks to define the areas and activity that drive ET-like tremor while also directly challenging the existing theories using a combination of genetic, pharmacological, electrophysiological, and optogenetic approaches. A Cre/loxP genetic approach was used to conditionally remove the vesicular GABA transporter (VGAT) specifically from Purkinje cells, resulting in ?silenced? Purkinje cells that cannot communicate with their downstream partners, the cerebellar nuclei. Preliminary data suggest that the loss of Purkinje cell signaling does not cause tremor, contrary to theories that suggest Purkinje cell loss or GABA receptor reduction lead to tremor. Data has also been collected showing that pharmacologically forcing inferior olive oscillations, which normally induces tremor in control mice, is ineffective in mice without Purkinje cell signaling. Characterization of Purkinje cell and cerebellar nuclei activity in actively tremoring mice has begun to uncover critical clues for how the cerebellum might initiate tremor. This proposal expands on these data by testing the hypothesis that a unique pattern of Purkinje cell misfiring causes ET-like tremor by promoting 10Hz activity throughout the motor system. The first aim tests how Purkinje cell signals facilitate tremor. In vivo awake electrophysiology and genetic approaches will be used to determine the specific firing patterns of Purkinje and cerebellar nuclear neurons that propagate tremor and whether there is a developmental component to susceptibility to tremor. The second aim tests how the cerebellum interacts with other areas of the motor system, including thalamus and motor cortex, during tremor. This will determine whether the effects of Purkinje cell silencing extend outside the cerebellum to other motor areas, particularly when under conditions that normally result in tremor. Preliminary data suggests that Purkinje cell silencing blocks the propagation of tremor. Therefore, pharmacological methods will be used to determine if the Purkinje cell has a unique and privileged position in the tremor circuit. Optogenetics will also be used to mimic tremor signals by driving specific patterns of activity in the Purkinje cells. These experiments will be used to directly pinpoint the cell types and activity patterns that are sufficient to trigger tremor signals in the motor system and to determine how of these cells drive pathological oscillations in downstream areas.